1. Field of the Invention
The present invention relates to the field of hearing aids. The invention, more specifically relates to hearing aids and methods utilizing in-situ occlusion effect or in-situ directly transmitted sound measurement. In addition, the invention relates to a method for vent size determination, a method for fitting a hearing aid based on measured in-situ occlusion effect, and a hearing aid with a customized ear plug.
2. Description of the Related Art
The occlusion effect is a well-known problem for hearing aid users. When someone speaks, sound is likely to propagate through bone conduction to the inside of the ear canal. The sound pressure level at the ear drum due to the person speaking is likely to increase on occluding the ear canal relative to the un-occluded ear canal, since the sound cannot escape the open ear anymore.
The occlusion effect is therefore also described as the low frequency boost of own voice that occurs when the ear is occluded. A user may thus perceive his or her own voice as hollow or booming, which in particular is annoying if the hearing loss is small in the low frequencies. Typically, the occlusion effect is alleviated by drilling a ventilation canal in the ear plug or shell. The larger the ventilation, the less occlusion effect remains. In today's hearing aid fitting situations, the decision on the vent size lies entirely by the dispenser, and is based on good judgment and rules of thumb. The amount of occlusion effect, which depends on the individual ear and the vent size, is only qualitatively assessed in fitting today. Once the ear plug for the user has been created, the dispenser, receiving the complaint of the user, can only advise the user to get used to the occlusion effect or offer drilling a larger hole through the plug. However, in particular for CIC and ITE hearing aids, drilling a larger vent is not possible, and would therefore demand the production of an entirely new hearing aid. It is therefore important to determine the right vent size in the first guess, demanding much experience in the field.
Usually the occlusion effect is remedied by venting without knowing an exact value for an appropriate vent size to just attenuate the low frequency part of any sound source within the ear.
U.S. Pat. No. 6,766,031 discloses an in-the-ear hearing aid wherein occlusion effect is defeated by providing a vent.
U.S. Pat. No. 7,031,484 discloses a hearing aid wherein the occlusion effect is countered by tuning the compressor to suppress the gain in low frequencies.
Regarding vent size determination, it is the standard practice when ordering a custom plug to decide on the vent size based on rules of thumb developed through experience. The plug will then be manufactured by, for example, a rapid prototyping method including a vent with a diameter as ordered. By the current practice it is therefore not possible to predict the occlusion effect very well.
Another important acoustic property of an ear plug is the propagation of sound from the outside and directly, i.e. not amplified by the hearing aid, into the inner part of the ear canal, which is called directly transmitted sound. Directly transmitted sound may interfere with signals output by the hearing aid causing a decrease of the speech intelligibility and overall sound quality for the user.
Thus, there is a need for improved hearing aids and methods for determining the occlusion effect and other acoustic effects as well as for fitting a hearing aid.